ppc4xx: Add CFG_POST_ALT_WORD_ADDR to support non OCM POST WORD storage

The privious 4xx POST implementation only supported storing the POST
WORD in OCM. Since we need to reserve the OCM on LWMON5 for the logbuffer
we need to store the POST WORD in some other non volatile location.
This patch adds CFG_POST_ALT_WORD_ADDR to specify an address for such
a location.

Signed-off-by: Stefan Roese <sr@denx.de>